Panel light fixtures are typically designed to take into account the light distribution, intensity, and thermal characteristics of the source. Panel light fixtures have historically been incandescent light bulbs or fluorescent light bulbs. A wide range of reflectors and optical devices have been developed over the years to generate a particular output distribution and/or deliver maximum efficiency for an incandescent light bulb.
Fluorescent light bulbs work differently than incandescent light bulbs. An incandescent light has electricity pass through a filament, which emits light. A fluorescent light is a gas discharge light where electricity excites mercury vapor, which emits ultraviolet light. The ultraviolet light strikes phosphors in the fluorescent light, which in turn emit visible light. Fluorescent light bulbs have the added need of ballasts or other electronic methods of converting the available power into a useful form. Fluorescent light bulbs use different reflectors and different optical devices from an incandescent light bulb to achieve a similar result of a particular output distribution and/or maximum efficiency for a fluorescent bulb.
A new light source based on a distributed array of light emitting diodes (LEDs) within a solid luminescent element has been disclosed by Zimmerman et al. in U.S. Pat. No. 7,285,791, commonly assigned as the present application and herein incorporated by reference. Electricity passes through an active region of semiconductor material to emit light in a light emitting diode. The solid luminescent element is a wavelength conversion chip. U.S. Published Patent Applications 20080042153 and 20080149166, commonly assigned as the present application and herein incorporated by reference, teach wavelength conversion chips for use with light emitting diodes. A light emitting diode, such as those in U.S. Published Patent Applications 20080182353 and 20080258165, commonly assigned as the present application and herein incorporated by reference, will emit light of a first wavelength and that first wavelength light will be converted into light of a second wavelength by the wavelength conversion chip.
A panel light source can be made in a variety of shapes and output distributions ranging from directional to isotropic using thermally conductive luminescent elements. Power conditioning and control electronics can also be incorporated into the bulb itself because the thermally conductive luminescent element is a solid. A variety of means can also be used to connect to the available power source. In addition, the distributed nature of the sources allows for cooling via natural convection means as long as sufficient airflow is allowed by the light fixture eliminating or greatly reducing the need for additional heatsinking means. It also provides a substrate for integration of solar and energy storage means.
In most cases, existing LED light sources are based on high intensity point sources, which required extensive thermal heatsinking to operate and distribute the heat generated in the point sources over a large area. The localized nature of these high intensity point sources dictate that large heatsinks must be used especially in the case of natural convection cooled applications. While 100 lumen/watt performance levels have been demonstrated for bulbs outside the fixture, performance can degrade as much as 50% once this type of solid state light source is used inside the fixture due to airflow restriction and lack of ventilation. This is especially true for the cases where fixtures are surrounded by insulation, as is the case for most residential applications. The heatsinks typically required to cool these high intensity point sources are both heavy and present a hazard especially in overhead lighting applications, where a falling light could severely injure a passerby. Additionally, the fact that the source is so localized means that some type of distribution or diffusing means must be used to deal with the brightness level generated. This is required from an aesthetic and safety point of view. The small nature of the source means that imaging of the source on the retina of the eye is of great concern. This is especially true for UV and blue sources due to additive photochemical effects. In general, brightness levels greater than 5,000 to 10,000 FtL are uncomfortable for direct viewing especially at night. High intensity point sources can be several orders of magnitude higher brightness than what can be comfortably viewed directly. Lastly, the localized nature of the heat source generated by these high intensity point sources dictate that high efficiency heat sink designs must be used which are more susceptible to dust and other environmental effects especially in outside applications. This dictates periodic maintenance of the light sources, which is impractical in many cases. The need therefore exists for improved fixtures that can provide directional control, allow cooling of the sources, and safely illuminate our homes and businesses. Panel lights based on thermally conductivity luminescent elements are disclosed which enable new types of light fixtures and are ideally suited for general illumination applications.